In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, large antenna arrays have been introduced at cellular radio access network node and other wireless access points and have become a viable option to boost capacity and user data rates in the communications network. A radio access network node (RANN) or an access point (AP) equipped with excessive number of antennas, can simultaneously schedule multiple wireless devices, such as user equipment (UE) or stations (STA) at the same time/frequency interval with simple linear processing such as maximum-ratio transmission (MRT) or zero-forcing (ZF) in the downlink (DL), i.e. during communications from the RANN/AP to the wireless device, and maximum-ratio combining (MRC) or ZF in the uplink (UL), i.e., during communications from the wireless device to the RANN/AP. This is often referred to as very-large (or full dimension, FD) multiple-input multiple-output (VL-MIMO) or massive MIMO. Gains with VL-MIMO are achieved without consuming any additional spectrum. Additionally, the radiated energy efficiency with VL-MIMO can be substantially improved.
One non-limiting possible usage of FD MIMO technologies is (extreme) narrow beam forming for DL transmissions, that enables a RANN/AP to focus the transmitted energy to the desired wireless devices and thereby boosting the coverage and user data rates for DL transmissions.
For communications networks based on VL-MIMO it is not trivial how to acquire channel state information (CSI) in a scalable fashion. CSI is acquired for gaining performance potentials of the used excessive amount of transmit antennas. Traditionally, each wireless device continuously measures on the pilot (reference) symbols transmitted by the RANN/AP during the DL transmission phase to estimate the downlink channel gain and feeds it back to the RANN/AP via a reverse link during the UL transmission phase.
Since the number of required pilots in the downlink is proportional to the number of antennas at the RANN/AP, feedback based schemes are not scalable. Existing mechanisms for addressing this issue are based on operations performed in the time-division duplex (TDD) mode and rely on the channel reciprocity between the uplink and the downlink. More precisely, each wireless device transmits sounding reference signals (SRSs) in the uplink phase. These SRSs are then used by the RANN/AP to estimate both the uplink and downlink wireless channel. The number of uplink pilot signals used by RANNs/APs in such communications networks is proportional to the number of wireless devices scheduled in the same time frequency resource, which is typically significantly smaller than the number of antennas at the RANN/AP.
In an FD MIMO system there may be no need for DL demodulation reference signal (DMRS) since the channels towards the wireless devices are pre-equalized at the RANN/AP using proper precoders, such as maximum ratio transmission (MRT) or zero forcing (ZF). In a practical implementation downlink DMRS can still be present due to other considerations.
In existing systems, wireless channel sounding refers to the mechanism that enables a RANN or AP to obtain wideband channel state information in parts of the spectrum in which no wireless data transmission necessarily is taking place. Specifically, in cellular systems, a RANN has two main usages of wideband channel sounding. Firstly, to acquire UL channel state information in frequency and time resources in which a wireless device is currently not scheduled (even though the wireless device may currently use other parts of the spectrum). Secondly, to acquire UL channel state information of wireless devices that are currently not transmitting uplink data.
In existing systems, demodulation reference signals (DMRS) are used to enable coherent demodulation of the transmitted data. More precisely, the DMRS is inserted in-band with the data so that it goes through the same processing chain as does the data. This enables coherent demodulation of the data. Herein, the data includes any type of information to be communicated including DL payload data (transmitted for example in a Long Term Evolution (LTE) physical downlink shared channel—PDSCH), UL payload data (transmitted for example in an LTE physical uplink shared channel—PUSCH), DL control signaling (transmitted for example in an LTE physical downlink control channel—PDCCH), and UL control signaling (transmitted for example in an LTE physical uplink control channel—PUCCH).
FIG. 9 illustrates exemplary UL grants 910 and DL assignments 920 on a time frequency grid 900 for a wireless device according to a state-of-the-art system operating in TDD. In FIG. 9, the DL transmission phase and the UL transmission phase are illustrated. Additionally in this figure, the subset of frequency intervals that are assigned to a specific wireless device during DL and UL transmission phases is illustrated. The DL assignment and UL grant need not include the same frequency intervals and might overlap in some parts, as schematically illustrated in FIG. 9. In FIG. 9, DL DMRS 940a, 940b and UL DMRS 930a, 930b assignments are also illustrated. In some scenarios, there might be a sounding reference signal transmitted during the UL phase as described in above.
The wireless device may be equipped with more than one transmit and/or receive antennas. In such cases, it is possible to transmit more than one data-stream to the wireless device, thereby exploiting the additional degrees of freedom offered by having more than one antenna at the wireless device. This is often referred to as multi-layer transmission. In general terms, the number of possible data-streams in a MIMO system with nt number of transmit antennas and nr number of receive antennas is min(nt, nr), where min(nt, nr) denotes the minimum of nt and nr.
In the current state-of-the-art schemes the UL DMRS and SRS are orthogonal resources. Hence, when the reference structures for UL DMRS and SRS are dual purpose, the assignments of pilot signals are not optimal if the current state-of-the-art orthogonal structure is used for assigning UL reference signals. This is due to the following two main reasons. Firstly, assigning two distinct orthogonal sequences for SRS and DMRS means that two orthogonal sequences per wireless device are needed and hence causes inefficient usage of orthogonal resources. Secondly, since some channel resources are needed for the transmission of DMRS and SRS, unnecessary pilot transmissions would result in inefficient usage of channel resources.
Hence, there is still a need for an improved assignment of uplink pilot reference signals.